What is AdSense Client ID?

Your publisher ID is the unique identifier for your account. Google Adsense ID is known as Adsense client ID or Adsense Publisher ID, which is unique to each Adsense publisher.

To protect the security of your account and make it easier for us to find account-specific details, you may be asked to provide this ID when you communicate with Google

To find your unique AdSense publisher ID number, log into your AdSense account at http://www.google.com/adsense. You may see your ID one the upper right of the page like the image. Or, on the Account settings tab, scroll down to the Account information section. Your publisher ID for each AdSense product and feature will be located in this section.

Alternatively, you can find your publisher ID number within your AdSense code. Just locate the AdSense code in the HTML source code of your website, and you'll spot your publisher ID in a line like the one shown below:
google_ad_client="pub-xxxxxxxxxxxxxxxx";